1 package day03;
 2 
 3 public class UpTest {
 4     public static void main(){
 5         Aoo o = new Aoo();
 6         o.a = 3;
 7         o.b = 3;  //错误,不能访问子类成员
 8         
 9         Boo x = new Boo();
10         x.b = 3;
11         x.a = 3;
12         x.say();
13         
14         Aoo y = new Boo();  //向上造型，父类引用指向子类对象
15         y.a = 3;
16         y.b = 3;
17         
18     }
19 }
20 
21 class Aoo{
22     int a;
23     void show(){
24     }
25 }
26 
27 class Boo extends Aoo{
28     int b;
29     void say(){}
30 }